A large number of pharmaceutical substances for various purposes have been developed for introduction into animals, including humans. The substances include therapeutic agents, such as drugs; prophylactic agents, such as antigens for use in vaccines; and diagnostic agents, such as labeled imaging agents. The substances may be introduced by a variety of enteral and parenteral modes of administration.
There has recently been a proliferation of potential and realized pharmaceutical compounds that are macromolecules, such as proteins and nucleic acid molecules. These macromolecular compounds present particular problems for drug delivery, since they tend to be unstable, poorly absorbed, and easily metabolized.
There has also been renewed interest in the mucosal administration of pharmaceutical substances. The mucosa refers to the epithelial tissue that lines the internal cavities of the body, such as the gastrointestinal tract, the respiratory tract, the lungs, and the genitalia. For the purpose of this specification, the mucosa will also include the external surface of the eye, i.e. the cornea.
Some common modes of mucosal administration include oral and nasal administrations. Currently known methods of ocular administration are subject to several limitations that compromise their effectiveness. These problems include rapid nasolacrimal drainage, poor corneal penetration, non-productive conjunctival loss, and unwanted systemic exposure.
Almeida et al. have reviewed the mucosal administration of vaccines in general, and nasal administration of vaccines in particular in the Journal of Drug Targeting 3, 456-467 1996). Mucosal immunity is based on the existence in the mucosa of mucosal-associated lymphoid tissue (MALT). These include gut-associated lymphoid tissue (GALT), bronchus-associated lymphoid tissue (BALT), and nasal-associated lymphoid tissue (NALT). Mucosal immunization is capable of inducing both a local (IgA) and systemic (IgG) immune response. In addition, there is a common mucosal immune system, whereby an antigen enters the MALT at a local site, and is transported through the regional lymph nodes to other mucosal surfaces, where an immune response is also induced.
Pharmaceutical substances may be administered either in the absence or in the presence of a carrier. Various purposes may be served by such carriers, such as the controlled release of biologically active molecules, and the targeting of biologically active molecules to specific tissues.
Illum et al. investigated three microspheres as potential nasal drug delivery systems. The microspheres were albumin, starch, and DEAE-Sephadex. Although these microspheres showed some promise, certain problems still need to be overcome.
For example, Illum et al. reported that the size of the microspheres must be greater than 10 .mu.m. Such large particles, however, have certain disadvantages. For example, they cannot be sterilized by ultrafiltration, requiring other methods, such as the use of preservatives.
In addition, Illum et al. reported difficulty releasing drugs from microspheres having a cationic charge. There are advantages to positively charged microspheres, and the problems reported by Illum et al. must be overcome.
Liposomes are often used as carriers for substances. They have shown potential as controlled release drug delivery systems and as immunological adjuvants. The use of liposomes as carriers for vaccines is discussed in the article by Almeida et al. mentioned above. More specifically, the use of liposomes as carriers for influenza vaccines was discussed by El Guink et al., Vaccine 7, 147-151 (1989), and in U.S. Pat. No. 4,196,191 of the Burroughs Wellcome Company and International PCT Application WO 92/03162 of the Wellcome Foundation.
There are, however, disadvantages in the use of liposomes as carrier for active compounds. For example, only small amounts of one compound can generally be incorporated in a liposome, and the ratio of active compound to lipid is low. Moreover, the active compound is often released too early.
Liposomes also present certain manufacturing disadvantages. For example, detergents and solvents are used to increase solubility during one phase of the manufacturing process. These detergents and solvents must be eliminated from the drug at a later stage.
Other difficulties in using liposomes as drug delivery systems have been reported by Meisner in Chapter 3, page 31 of Pharmaceutical Particulate Carriers--Therapeutic Applications, A. Roland, ed., Marcel Dekker, 1993. There is, therefore, the need for a more flexible carrier for substances.
Other carriers for substances have been described in U.S. Pat. No. 4,921,757 and 4,900,556 of the Massachussets Institute of Technology; U.S. Pat. No. 5,354,853 of Genzyme Corporation; and European Patent 352 295 of Access Pharmaceuticals, Inc. For example, the Access patent describes a carrier for drugs and diagnostic agents having a multivalent binding agent, such as heparin. The multivalent binding agent is specific for endothelial surface determinants, and may be as large as three micrometers.
The carriers described in the Access patent have, however certain disadvantages. First, the Access carriers bind specifically to endothelial cells. Also, the Access patent describes only carriers pre-loaded with the drug or diagnostic agent prior to administration. Such methods can lead to instability. Thus, Examples X and XII on page 19 of the Access patent measure stability in hours. Also, the carriers described in the Access patent are generally too large to be subjected to microfiltration.
In addition to those mentioned above, numerous other microspheres and nanospheres are known. These include polyacrylate, latex, and polylactide polymers. Bjork and Edman, International Journal of Pharmaceutics 47, 233 (1988) reported that starch, cellulose, and dextran microspheres can act as absorption enhancers if they satisfy certain criteria, i.e., they must be water absorptive, water insoluble, and administered in powder form to the nose.
A new type of improved carrier was described by Biovector Therapeutics, S.A. in International PCT Application WO 94/20078. These carriers, called Supramolecular Biovectors (SMBVs) act as solvated suspensions in water, while still maintaining their integrity as substance-encapsulating particles. These SMBVs comprise a non-liquid hydrophilic core, such as a cross-linked polysaccharide or a cross-linked oligosaccharide and, optionally, an external layer comprising an amphiphilic compound, such as a phospholipid. The Biovector optionally has cationic or anionic ligands grafted into the polysaccharide or oligosaccharide core. The Biovector also optionally contains a layer of lipid compounds grafted onto the core by covalent bonds. See International PCT Application WO 94/23701. These Biovectors have been described as being useful in vaccines, such as in CMV vaccines. See International PCT Application WO 96/06638.
There is a need for a carrier that is capable of delivering substances to animals, including humans, efficiently, and that avoids the disadvantages of prior art carriers. An object of the present invention is to provide a method for the administration of biologically active molecules and other substances to mammals in a way that avoids the disadvantages discussed above. More specifically, an object of the present invention is to provide a method for administering substances to mammals by means of a carrier that directs the substance to the mucosa in a non-specific manner, that is capable of being loaded with the substance immediately prior to administration, that is of a size susceptible to microfiltration, and that is stable for up to twelve months and even one or more years.